The present invention relates to a circuit for correcting the output phase of a rotary encoder and, more particularly, to a circuit suitable for correcting a variation in phase of an encoder output pulse due to the eccentricity of a coding disc.
A rotary encoder is used for detecting the rotation angle of a rotating shaft. For example, it constitutes a main part of a synchronizing signal generator in a line printer or the like. A typical structure of the rotary encoder includes a coding disc mounted on a rotating shaft and provided with a multiplicity of marks like slits of a stroboscope, a light detector disposed in the vicinity of the disc, and a counter for counting a train of pulses derived from the light detector, the counter being reset by a signal indicative of the detection of a mark in the disc representing a reference position of the disc.
In the encoder having the above-mentioned structure, a phase error may be included in the output pulse of the encoder. It is said that the phase error is mainly caused by the eccentricity of the coding disc due to a deviation of the center axis of the coding disc from that of the rotating shaft of the encoder. This eccentricity appears as a phase error of a rectangular pulse waveform which is the output of the encoder. This error is an obstacle to accurate detection of the rotation angle of the rotating shaft. Usually, the phase error due to the eccentricity of the coding disc assumes a spurious sinusoidal waveform the period of which corresponds to one rotation of the coding disc.
An example of a rotary encoder capable of correcting such a phase error has been disclosed in JP-A-60-146113 laid open in 1985. In the disclosed rotary encoder, two light detectors are oppositely arranged at an angle of 180.degree. relative to each other around the center axis of a rotating shaft. Variations in phase of the output pulse signals derived from the two light detectors due to the eccentricity of a coding disc appear as sinusoidal variations which have their phases reverse to each other. By summing these two light detector output signals, it is possible to cancel an error in the output of the encoder, thereby allowing accurate detection of the rotation angle. This principle holds for even the case where more than two light detectors are used. In general, as the number of light detectors used increases, more precise correction can be correspondingly made. However, such a rotary encoder using a plurality of light detectors has the problems that it is difficult to make the size of the encoder small since it is difficult to incorporate many light detectors into the encoder the excess space of which is limited and that troublesome and complicated adjustments including a phase matching between the light detectors are required.